diff -r 8f37d772f71f Lib/http/client.py
--- a/Lib/http/client.py	Wed Aug 10 13:16:26 2016 -0400
+++ b/Lib/http/client.py	Wed Aug 10 19:58:50 2016 +0200
@@ -1150,8 +1150,10 @@
             raise CannotSendHeader()
         self._send_output(message_body)
 
-    def request(self, method, url, body=None, headers={}):
+    def request(self, method, url, body=None, headers=None):
         """Send a complete request to the server."""
+        if headers is None:
+            headers = {}
         self._send_request(method, url, body, headers)
 
     def _set_content_length(self, body, method):